Effect of texture and microstructure of zirconium alloys on their oxidation and oxide texture

Zirconium oxide is a barrier against corrosion. The oxidation behavior of zirconium alloys is believed to be related to the properties of the substrate and the zirconium oxide grown on this substrate. Both monoclinic and tetragonal phases of the oxide grown on Zr-2.5Nb and Zircaloy-4 are strongly textured. The volume fraction of tetragonal phase in zirconium oxide was investigated. The tetragonal phase percentage gradually decreases with an increase of the oxide thickness.The oxidation kinetics of Zr-2.5Nb and Zircaloy-4 was investigated and the results demonstrate that the oxidation rate is related to the volume fraction of tetragonal phase. No texture evolution was observed in both alloys as the oxidation time varied. It was found that the higher the volume fraction of tetragonal phase, the higher oxidation resistance.The texture formation of the oxide is related to the texture of the substrate. A model of crystal orientation relationship between alpha-Zr and the monoclinic phase was applied to explain the formation of oxide textures on substrates having different textures. Good agreement between the results predicted from the model and the experimental results were obtained. A crystallographic orientation relationship between the monoclinic and tetragonal oxide phases is proposed based on the texture investigation of both phases. Various methods were applied to modify the texture of Zr-2.5Nb. Heat treatment at 560°C caused decomposition of the beta-Zr phase. This heat treatment only slightly increased the sharpness of texture of the alpha-Zr phase and heat treatment improved the oxidation resistance of Zr-2.5Nb. The improvement is attributed to a change of morphology of the beta-Zr phase. High temperature heat treatment at 855°C significantly changes the microstructure and texture of the substrate. A needle shape second phase formed inside the alpha-Zr phase is responsible for the increase of oxidation rate. Cold deformation causes the texture in the Zr-2.5Nb substrate to change from (1120)[1100] to (11 23)[1100]. The change of the texture degrades the oxidation resistance of Zr-2.5Nb